The proposed research is part of a program in comparative immunohematology which is investigating the development of cells involved in immunity in chickens. Two aspects are currently receiving our attention - (1) an analysis of the origin, nature, and subsequent fates of antigen-reactive units which appear in the general circulation shortly after immunization with heterologous erythrocytes, and (2) the effects of various potentially immunosuppressive agents on the development of these cells. The adoptive cell transfer technic, the "in vivo" culture model, is a key method for the detection of blood-borne progenitor of antibody-forming cells, for the assay of immunologic competence and for setting up experiments in cell interaction and reconstitution studies. Although some success has been obtained earlier with the allogeneic in vivo model, histocompatible donor and hosts are desirable to avoid the complications of the graft-versus-host and host-versus-graft reactions. We are presently developing a colony of inbred White Leghorn chickens of known blood group type and testing their suitability for cell transfer experiments. Results of preliminary studies are very encouraging.